The processes for biologically removing nitrogenous compounds from waste waters generally include two steps: nitrification and denitrification. Nitrification is an oxidation reaction, by autotrophic bacteria, of ammoniacal nitrogen into nitrous or nitric nitrogen. The autotrophic bacteria responsible for this conversion are bacteria which grow only in the presence of oxygen. The second step, denitrification, consists of a reduction to nitrogen gas, by means of denitrifying heterotrophic bacteria, of the nitrites or nitrates produced during nitrification reactions. The denitrifying heterotrophic bacteria responsible for the conversion of nitrites or nitrates into nitrogen gas are facultative aerobic bacteria, which grow either in aerated medium in the presence of oxygen, or in anoxic medium in the absence of free oxygen, but in the presence of the constituent oxygen of nitrates and/or nitrites.
The processes for removing nitrogenous compounds from waste waters must thus include two different zones: an anoxic zone or step for the denitrification and an aeration zone or step for the nitrification.
In the water treatment sector, two forms of bacterial population can be used to carry out the biological removal of nitrogen: free cultures or bound cultures.
FIG. 1 in the attached drawings diagrammatically represents a conventional system for removing nitrogen during the treatment of waste waters. In such a system, the free culture, for example an activated sludge, is a mixture of different microorganisms in suspension, in the form of flocs. Thus, the nitrifying autotrophic bacteria are present in the same floc as the denitrifying heterotrophic bacteria and many other microorganisms. The set of microorganisms in the floc runs throughout the system, i.e. the anoxic zones 2 and aerated zone(s) 9, to be separated from the treated water 10 by decantation in the clarifier 6 and recycled 7 into the top of the system. The degree of recycling of the nitrified liquor 7 into the anoxic zone 2 is often between 100 and 600%, this value depending either on the concentration of the organic carbonate available in the waste water, or on the minimum admissible residence time in the anoxic zone 2. The major drawback of such a known configuration lies in the fact that the nitrifying autotrophic bacteria are obliged to cross the anoxic zone, in which, in the absence of oxygen, they do not reproduce.
Similarly, the denitrifying heterotrophic bacteria must cross the aerated zone, while using the dissolved oxygen as the final electron acceptor instead of nitric or nitrous nitrogen. These repeated passages in aerated medium generally have the consequence of temporarily deactivating the enzymes involved in the denitrification reactions, which is reflected by a lag time at the start of the anoxic period, corresponding to a reactivation of these enzymes. Consequently, the reaction system thus involved is not optimal and reduces the kinetics and the efficacy of the processes for removing nitrogen by free cultures.
Furthermore, the limiting step of a conventional process involving nitrifying/denitrifying activated sludge is the nitrification, since the autotrophic bacteria responsible for the conversion of the ammoniacal nitrogen into nitric and/or nitrous nitrogen have very slow growth rates. The minimum sludge age to allow a satisfactory nitrification must thus be fairly high, which has a negative effect on the denitrifying heterotrophic bacteria, which are, in contrast, most active at very low sludge ages and at very high organic loads. Thus, these so-called conventional systems are known to those skilled in the art as being processes containing a low applied load.
Moreover, it is well known by those skilled in the art that these systems are highly sensitive to load surges, but both as regards the risks of water overload (risk of loss of sludge from the decanter) and biological overload (partial inhibition of nitrification due to an excess of carbon-based pollution or to a lack of aeration).
Unlike the free cultures, the method for using the bound cultures, as illustrated by the scheme in FIG. 2 of the attached drawings, makes it possible to separate the nitrifying autotrophic populations 9 and denitrifying heterotrophic populations 2. Thus, the bacteria immobilized on a solid support are permanently in a more favorable medium, without periodic passage under conditions in which their activity would be reduced. The result of this type of implementation is to appreciably increase the kinetics of the biological reactions.
The ideal solution thus consists in separating the autotrophic and heterotrophic populations. Hitherto, this separation has only been carried out for bound cultures (Water Science and Technology, Vol. 19, 139-150, 1987; EP-A 0 293 521). As regards the sector of free cultures, the use of an activated sludge which is permanently evolving in anoxic medium, in the absence of dissolved oxygen, makes it possible to maintain a denitrifying biomass of very high activity. This fact, which is already known in the water treatment sector (Korrespondenz Abwasser, Vol. 41, No. 11, pp. 2077-2081, 1994) has, however, never resulted in an industrial application, mainly due to failure in controlling the conditions of the anoxic medium and of the degree of recycling of the nitric nitrogen.
EP-A-0 442 337 describes a biological purification process comprising three reactors in which the biomass is identical. In this prior publication, the activated sludge has a low load and it satisfies all functions: nitrification, denitrification and removal of carbon. In this prior publication, there is no separation of the heterotrophic and autotrophic biomasses.
JP-56-002,892 describes a biological treatment process comprising an anoxic (denitrification) tank coupled to the aerobic (nitrification) tank with no separation of the biomasses, the sole biomass carrying out all the functions: nitrification, denitrification and removal of carbon. Here also, this is a matter of a conventional activated sludge with a low load.